1. Field of the Invention
This invention relates to the field of electronics, and in particular to electronic systems that utilize isolated power-bus systems.
2. Description of Related Art
Isolated power-bus systems are commonly employed to isolate a component system from a power source. For example, electronic devices often include a power transformer that provides a dual purpose of voltage step-down and voltage isolation from a mains power source. By isolating the component power supply from the primary voltage source, the bus wires are insensitive to shorts from a bus wire to the supply voltage and ground potentials.
Some systems, such as radios, televisions, and the like, operate substantially independently. Many systems, however, operate in communication with other devices and systems, some of which may be operated from the primary power source. Communications among devices, however, creates a current path between the two devices. Unless the data path between the two devices is balanced perfectly, current travels from one device to the other. If one of the devices is in an isolated system, and the other device is in the primary, non-isolated, system, the current will typically return via a high-impedance parasitic path between the two systems. Although the difference current associated with a communications signal is typically very small, the high-impedance path causes a common-mode voltage on the isolated components, and this common-mode voltage potential is a source for electromagnetic interference (EMI). Providing a low-impedance return path between the component on the isolated bus and the device on the primary supply system can substantially reduce this common-mode voltage, but such a low-impedance path will defeat the purpose of the isolation bus. A short between the wires of the isolated system and the primary supply system can result in a significant amount of short circuit current through the xe2x80x9cisolatedxe2x80x9d component and bus, via the low-impedance return path.
It Is an object of this invention to reduce the common-mode voltage potential on an isolated bus. It is a further object of this invention to reduce the common-mode voltage potential on an isolated bus without substantially affecting the isolation provided.
These objects and others are achieved by providing a current-limited low-impedance return path between an isolated bus system and the primary supply that provides this isolated bus system. In a preferred embodiment, a voltage follower circuit with an input at ground potential supplies a controlled zero-voltage to a center-tap on the load side of an isolation transformer. This voltage follower circuit provides a low-impedance path for unbalanced communications signals, but is configured to present a high-impedance, or current-limited, path to short-circuit currents. In this manner, the commonmode voltage changes that are caused by parasitic currents between the primary supply and the isolated bus system are minimized by providing the low-impedance path for these currents, and the isolation is maintained by providing the high-impedance path for short-circuit currents.